Endemic goiter is an important public health problem in Turkey. Legislation for mandatory iodization of household salt was passed in July 1999. Current study is aimed at ascertaining the goiter prevalence and iodine nutrition in school-age children (SAC) living in known endemic areas of Turkey. Sonographic thyroid volumes (STV) and urinary iodine concentrations (UIC) of 5,948 SAC from 20 cities were measured between 1997-1999. STV of 31.8% of the SAC examined stayed above the upper-normal limits for the same age and gender recommended by the World Health Organization (WHO). Goiter prevalence ranged between 5 to 56% and median UIC ranged between 14 to 78 microg/l, indicating severe to moderate iodine deficiency (ID) in 14 and mild ID in 6 of the cities surveyed. Neither of the cities was found to have sufficient median UIC levels. The current study shows that endemic goiter is an important public health problem and iodine nutrition is inadequate nationwide. It also provides reliable scientific evidence and shows the need for a controlled and effective iodine supplementation program nationwide. Mandatory iodization of household salt seems to be the essential measure taken for the moment, additional measures may be needed in the near future.